The invention is directed to a means (IWF) for the bidirectional connection of an ELAN and a wide-area network offering a connectionless service.
ELAN stands for emulated LAN as described, in particular, in the specification 94-0035R9, "LAN Emulation Over ATM: Version 1.0" of the LAN Emulation SWG Drafting Group of the ATM Forum of Jan. 6, 1995, Bill Ellington, editor. This is thereby an approach of the ATM Forum to the migration of current LANs to ATM networks. ATM thereby means "Asynchronous Transfer Mode", i.e. asynchronous data or, respectively, information transmission methods. LAN is an abbreviation of "Local Area Network". LANs are datagram-oriented local networks that are described in, among other references, the article by David D. Clark, Kenneth D. Progran and David P. Reed, "An Introduction to Local Area Networks" in Proceedings of the IEEE, Vol. 66, No. 11, November 1978, pages 1497 through 1517. LANs are also described in ISO/IEC TR 8802-1, "Overview of LAN-Standards". LANs offer a connectionless service, what is referred to as the MAC service. MAC thereby stands for "Media Access Control". By contrast to this connectionless service, ATM technology is connection-oriented. When the protocols of the higher layers developed for LANs are to be used in emulated LANs on the basis of an ATM network, the properties of the connectionless MAC service must be produced in this ATM network. The LAN emulation according to the aforementioned specification realizes the MAC service in the local ATM network and thus defines a single emulated LAN, called ELAN below. The standard LAN protocols such as LLC, TCP/IP, SPX/IPX or TP/CLNP can be used in this ELAN.
The LAN emulation supports the two most frequently employed LAN standards, namely Ethernet according to IEEE 802.3 and Token Ring according to IEEE 802.5, whereby three frame lengths are supported given token ring. The routing of an information packet within an Ethernet ensues on the basis of a destination MAC address handed over from a higher layer. For the description of the information path, token ring LANs employ what are referred to as route descriptors in the frame header. The frame can be conveyed to the destination within token ring LANs on the basis of such a descriptor.
What is meant below by MAC address is an address within an LAN that corresponds to the route descriptor in a token ring and to the MAC address in an Ethernet.
For emulation of an LAN in an ATM network, the LAN emulation must, among other things, resolve destination MAC address into destination ATM addresses, realize multicast and broadcast, i.e. a distribution of information to a plurality of or to all subscribers, as well as assure the transmission of LAN emulation frames in the proper sequence. The LAN emulation has a client-server configuration. The client side is called LAN emulation client LEC and the server side is called LAN emulation service. The LAN emulation service is composed of LAN emulation server LES, broadcast-and-unknown server BUS and LAN emulation configuration server LECS. The LAN emulation client receives the destination MAC address from a higher-ranking layer, for example the LLC layer, and must find the corresponding ATM address, in order to subsequently initiate the setup of a direct ATM connection to the destination by signaling. The signaling can thereby ensue, for example, according to the ITU-T Recommendation Q.2831/Q.2971. An LEC can be a software driver that must be installed at all stations that are directly connected to the ATM network, i.e. are connected by an ATM hardware circuit to an ATM switch. However, an LEC can also be realized within this ATM hardware circuit.
An LEC maintains a table with all MAC addresses that are reported in the emulated LAN, for example in the framework of a configuration, and with the corresponding ATM addresses. The communication between the LECen and the LES ensues according to an LAN emulation address resolution protocol that, conforming to the English designation LAN Emulation Address Resolution Protocol, is referred to as LE_ARP. When an LEC does not know the destination ATM address of a destination MAC address, then it sends an inquiry with the destination MAC address to the LES. Such an inquiry for address resolution is referenced LE_ARP request. When the LES can resolve the destination ATM address, it replies with LE_ARP response. When it cannot, it sends the request to further LECen.
When an LEC receives an address resolution response LE_ARP response, then it sets up an ATM-UBR connection to the ATM address contained therein and sends a unicast frame. UBR thereby denotes "Unspecified Bit Rate", i.e. indicates that the bit rate is not specified. A unicast frame is an information or, respectively, data packet with a single addressee. In the transmission of frames within an ELAN, a distinction is made between unicast frame to one receiver and multicast or broadcast frame to several or all receivers. An ATM-UBR connection is maintained for 20 minutes from the last transmitted frame so that further frames can be sent to the same receiver in a simple way. To this end, the variable C12 is referenced in point 5.1.1 of the LAN emulation specification. The destination ATM addresses of destination MAC addresses are stored for a certain length of time in the LEC with the assistance of a cash mechanism. When there is no connection to a destination LEC but the destination ATM address is known in the sender LEC, a sender LEC can set up a connection without address resolution request and send a unicast frame.
Multicast frames to a group of subscribers or, respectively, LECs and broadcast frames to all subscribers or, respectively, LECs are sent to the aforementioned BUS. The BUS maintains connections to all LECs in its emulated LAN and distributes the arrived frames to the addressees.
Every LEC can be signed on in the configuration as what is referred to as proxy-LEC. A proxy-LEC receives all address resolution requests LE_ARP request that an LES cannot resolve. A proxy-LEC also receives all multicast and all broadcast frames.
The advantage of ATM technology is to be seen, among other things, therein that direct connections with flexible bandwidth can be set up between the communication parties. Such direct connections guarantee minimum time delays and a high information transmission rate. This advantage of ATM technology is utilized in the LAN emulation for unicast frames.
The connectionless service or CLS service can be realized with various technologies (for example, DQDB, ATM, FR). The service is described in the ITU-T Recommendation F.812.
Known realizations are the switched multi-megabit data service (SMDS) described in specifications of Bellcore, SMDS Interest Group (SIG) and European SMDS Interest Group (ESIG), as well as the connectionless broadband data service (CBDS) defined in ETSI Standard 300 217 and ITU-T Recommendation I.364. CLS has become widespread worldwide due to these realizations. Systems that offer this service are built by many manufacturers in the field of telecommunication.
The service is envisioned for worldwide data communication. At every access to the CLS network, one or more CLNAP addresses according to E.164 (E.164 address) are assigned (CLNAP=connection network access protocol).
A destination CLNAP address (E.164 address) is attached (encapsulation) to an incoming unicast frame with static or dynamic allocation tables on the basis of its destination MAC address and the encapsulated frame (also called CLS packet) is conducted to this CLNAP address.
For better understanding, the addresses in the ELAN are called unicast or, respectively, multicast/broadcast MAC addresses; but individual and group addresses shall be referred to in the CLS network.
Each CLS packet is transmitted independently of the others in the CLS network. The networks sees to the proper sequence of the CLS packets.
Under certain conditions (see prETS 300 478, 300 479), the CLNAP PDUs are encapsulated in CLNIP PDUs (CLNIP=connectionless network interface protocol).
The handling of multicast traffic in the CLS network is realized in the following way. What are referred to as group address agents (GAAs) contain tables with the individual CLNAP addresses that belong to a CLNAP group address. Each CLS packet that has a group address as destination address is conducted to the corresponding group address agent. When encapsulation was carried out, the same group address resides in the fields "CLNAP destination address" and "CLNIP destination address".
The GAA resolves the group address of the incoming CLS packet into individual CLNIP addresses, generates copies of the original packet and attaches to corresponding individual address to each copy as CLNIP address.
The "CLNAP destination address" field remains unmodified, so that the receiver can learn about the original group address. The LAN emulation describes an individual emulated LAN. No solutions are currently known for the coupling of ELANs. For performance reasons, the mechanisms described in the LAN emulation for address resolution and for the realization of multicast/broadcast in the WAN region cannot simply be transferred.
Various concepts for connecting local ATM networks such as, for example, ELANs via a wide-area ATM network are known in the article, "Interconnect Emulated LANs with White Area ATM networks" by Peter T. P. Chang and Bill Ellington, ATM Forum Technical Committee of Nov. 29 through Dec. 2, 1994.
In a first concept, a plurality of ELANs are thereby connected to a wide-area ATM network, whereby the address resolution and the data transmission are undertaken via a single LES and a single BUS. This concept leads to an enormous traffic volume for the realization of the broadcast function. The address resolution delay times in such a network are extremely high.
A further concept provides that ELANs be respectively connected to a wide-area ATM network via remote bridges. Either all remote bridges are thereby connected to one another via permanent virtual circuits PVC or the remote bridges are dynamically connected to one another with the assistance of an ATM signaling upon employment of an address resolution server. The transmission possibilities are thereby limited by the transmission possibilities of the remote bridges and the bandwidth of the permanent virtual circuits between two remote bridges. The remote bridges are flooded with broadcast and unknown servers of remote ELANs insofar as the remote bridge thereof does not respectively know the address of the remote bridges allocated to the destination MAC addresses.
A further concept provides that, instead of remote bridges, brouters be provided, a mixture of bridge and router. In this case, these brouters fulfill the function of an LAN emulation bridge at the ELAN side and fulfill the functions of a router at the side of the ATM wide-area network. As a result thereof, the broadcast problems are reduced; however, a limitation of the transmission possibilities via the ATM wide-area network due to the transmission possibilities of the brouters and of the permanent virtual circuits continues to exist.
A further concept provides that the LES of the individual ELANs as well as the BUS of the individual ELANs be connected to one another by direct connections. This, however, leads to a great plurality of direct connections and to a high traffic volume between the LES and the BUS of the individual ELANs. The traffic volume thereby increases linearly with the plurality of connected ELANs.
A further concept provides that the LES of the individual ELANs as well as the BUS of the individual ELANs be connected to a higher-ranking LES or, respectively, to a higher-ranking BUS via direct connections. This, however, likewise leads to a great plurality of direct connections and to a high traffic volume. The multilayer nature of BUS and higher-ranking BUS or, respectively, LES and higher-ranking LES also leads to time delays.
All described concepts provide for the coupling of an ELAN to an ATM network.